1. Field of the Invention
The invention concerns arrays or matrices of photosensitive elements.
The standard way to make photosensitive elements is to design a network of row conductors and a network of column conductors with a respective photosensitive element at each intersection of a row and a column. By the network of rows, a selection is made of a row of photosensitive elements for which the output electrical signals have to be known. By the network of column conductors, a respective output signal is read for each of the elements of the selected row.
2. Description of the Prior Art
One method to enable the individual selection of a row, and individual reading on each column, consists in using the association of a photosensitive element as such and an access transistor at each dot of the matrix. The photosensitive element is a photodiode or a phototransistor (the latter having the advantage of greater sensitivity, i.e. it gives a greater electrical charge for the same illumination). This photodiode is connected by means of the access transistor to a column conductor of the matrix. This transistor is controlled by a row conductor (and all the photosensitive elements of the same row are thus connected to the same row conductor).
The drawback of this type of structure is that, at each dot of the matrix, it requires both a photodiode and an access transistor placed beside the photodiode. This structure is bulky because of the area taken up by the access transistor at each dot of the matrix.
To reduce the space factor, i.e. in fact, to increase the definition while housing a greater number of dots in a given area, structures have been proposed wherein the access transistor is eliminated and replaced by a capacitor used, firstly, to store the charges generated by the photodiode and, secondly, to insulate the photodiode from the column conductor when the row corresponding to this photodiode is not selected.
A structure of this type is described, for example, in the French Pat. No. 86 00716. The advantage of the capacitor, as compared with the transistor, is that it can be placed on top of the photodiode and not beside it, thus reducing the space factor.
The drawback here is that the charges stored at each dot can no longer be read as simply as when there is an access transistor which it is enough to make conductive in order to connect the photodiode to a column conductor of the matrix. With a structure having no more than one photodiode and one capacitor, it is the photodiode itself that has to be made conductive by directly biasing it for the brief instant for which the reading lasts, so as to remove the charges, stored in the capacitor, to the column conductor (or to bring a compensation charge for the stored charges through the column conductor).
It is important to note that, in principle, this type of structure prohibits the use of phototransistors instead of photodiodes for, unlike the photodiode which can be made conductive by simple direct biasing under low voltage, the phototransistor cannot easily be made conductive without action being taken on its base. Now, in principle, its base is floating (if it were not, we would be back with a complicated structure which was sought to be avoided). To make the phototransistor conductive in darkness without acting on the base, it is necessary to make its base-emitter junction go into avalanche mode by applying a high voltage to it between the collector and the emitter, and this is all the more undesirable as it would introduce considerable noise into the signal which is to be read.